Metallic wire or cable is widely used in the public switched telephone network to deliver signals to customers. Most of the metallic wire or cable consists of twisted copper pairs insulated with paper-pulp or polyethylene. The pairs are assembled into units and the units into cores covered by a protective sheath. The cables are placed in underground conduits, buried in trenches, or placed on poles or other aerial structures. The cables are therefore exposed to several different environmental conditions that may have a deleterious effect on transmission performance.
One such environmental condition affecting transmission performance is the build up of moisture in the cable. It is known in the art that a build up of moisture in metallic cable affects the electrical resistance of the copper pairs. In fact, the electrical resistance of the copper pairs is used to monitor moisture related transmission performance. Specifically, a properly performing copper pair exhibits an electrical resistance of 100 Megaohms (M.OMEGA.). As moisture builds up in the cable core, the electrical resistance of the copper wire pairs is lowered. In the public switched telephone network transmission performance has been found to be acceptable where the copper wire electrical resistance is between 30 and 100 M.OMEGA.. Furthermore, operation support systems are designed to generate alarms when the copper pair resistance drops to 15 M.OMEGA.. Depending on the need for moisture protection, the cable core may be filled with petroleum jelly, may contain no filling at all, or may be kept under positive air pressure.
Where air pressure is used to keep water vapor or moisture away from the copper conductors, dry air is pressurized by compressors located in central offices and distributed by a network of half-inch diameter pipes to the copper cables. These pipes are designed to meet requirements set forth in Bellcore document TR-TSY-000206, Technical Reference, Cable Pressurization Air Feeder Pipe, Issue 1, April 1985, based on a March 1984 AT&T specification known as CA3131 and consist of a hollow laminated aluminum--polyethylene structure. The TR-TYS-000206 calls for a pipe intended for an environment having a maximum service temperature of 140.degree. F. (60.degree. C. ) and a maximum working pressure of 10 psig (68.9 kPag). The air feeder pipe was required to have a nominal inside diameter of approximately 0.6 inches and nominal outside diameter of approximately 0.7 inches. The polyethylene is then laminated to the outer exposed surface of the pipe and used as a jacket providing moisture protection. An outer mechanical sheet is allowed for additional moisture protection.
Air pipe systems developed and installed in accordance with the AT&T specification have become inadequate in supplying dry air to an expanding cable network. As more transmission cable is installed, the need arises to install more air feeder pipes. However, duct space in the underground conduit systems used to install air feeder pipe is scarce. In fact, in some instances telephone companies have run out of duct space to install more air pipes. In addition, leaks from these aging air pipe networks have forced the telephone operating companies to look for new approaches for supplying dry air to telephone cables. The installation of air feeder pipes having larger diameters has been proposed as a possible solution to the duct space exhaustion problem. Because a larger diameter pipe would be able to deliver significantly more dry air than the one half inch CA3131 pipe, the feeder pipe network would therefore be able to satisfy air delivery requirements while using significantly less pipe. In other words, fewer larger air pipes can feed more transmission cables.
An all-plastic pipe constructed of low-density polyethylene (LDPE) pipe having a nominal 1-inch inside diameter and without an aluminum moisture barrier has been considered as a possible replacement for CA3131 feeder pipe. However, because a 1-inch LDPE pipe exposes a larger surface area to the environment than the CA3131 pipe, the 1-inch LDPE pipe would significantly add more water vapor or liquid water into the pressurized air stream and, subsequently, to the void space of the cable, thereby significantly increasing the risk of transmission failure. Consequently a system employing air pipes constructed only of LDPE would require circulating pressurized air at a higher velocity than was previously required. Thus, in addition to replacing the pipes, an air feeder system employing all-plastic LDPE pipes may also require replacement of already existing air compressors. In fact, air feeder systems that have been deployed using an all-plastic LDPE pipe required replacement of the entire feeder plant and the addition of more monitoring gauges.
Another approach to solving the problem has been to place a metal barrier between an inner and outer layer of high density polyethylene (HDPE). While such an approach provides adequate moisture protection, it has some drawbacks. First, a metal barrier adds weight to the pipe which increases the tendency of the pipe to warp and/or kink during its lifecycle. Second, a metal barrier increases the susceptibility of the pipe to lightning. Third, the metal barrier decreases the flexibility of the pipe. Fourth, the metal barrier makes installation more difficult because the pipe is more difficult to cut, thereby increasing installation costs. Finally, corrosion of the metallic barrier poses a significant long term reliability risk.
Accordingly, a material that could be used to construct a larger diameter all-plastic pipe while providing adequate moisture protection would be of utility. In addition to providing adequate moisture protection, a pipe constructed from plastic materials must not be cost prohibitive and must be able to withstand both physical and environmental stresses.